Platelet transfusion is an essential part of the management of the aplastic patient. In 1985 in the U.S., over 4.2 million units of platelets were used; a 50% increase since 1980. Post-transfusion failure develop in 30-50% of transfused patients. Currently the clinical management of PTA is unsatisfactory. Prevention of PTA offers the best hope. Current data indicates PTA is initiated by dendritic leukocytes contaminating platelet concentrates and not platelets themselves. In theory nonimmunogenic platelet concentrates should be feasible. Preliminary data suggests that 2 types of product can reduce immunogenicity of pooled multiple donor (MD) platelet: 1) leukocyte depletion and 2) UV irradiation. Other studies have indicated that single donor (SD) concentrates prepared by plateletpheresis cause less PTA than MD. This study compares the effect of no treatment, UV irradiation or leukocyte depletion of SD and pooled MD platelets on PTA in a prospective randomized blinded 6 arm study. Study subjects will include adult patients with acute nonlymphocytic leukemia. We will target leukocyte depletion to less than 5 X 10 6 WBC per transfusion. Commercially available filters (Pall RC-100 for red cells and PL-100 for platelets) will be used. Fenwall Labs will supply a UV irradiation system. Alloimmunization will be determined by the development of lymphocytotoxic antibody or antiplatelet antibodies. If one or more arms show decreased PTA we anticipate patients will require fewer platelet transfusions. We plan to register 26 patients/year x3 years. With 6-8 institutions participating we anticipate randomizing a total of 480 patients. This will be sufficient to detect a 50-66% reduction in PTA at alpha = .05, power = .8. Sample size should also allow detection of 20% decrease in transfusion needs. Pilot data on the effect of these modifications on decreasing transfusion reactions and evidence (seroconversion) of cytomegalovirus infection will also be sought. Procedures developed in this study should be generally applicable to clinical care.